A new map of Pluto, the most distant planet in the Solar System, shows a mysterious dark streak just south of its equator.

The map was constructed by watching Pluto's large moon, Charon, pass in front of the planet.

Pluto and Charon seen by the Hubble Space Telescope

From previous observations, it is known that Pluto is covered with a nitrogen frost with traces of carbon monoxide ice.

The reddish, dark streak may be a region where deposits of more complex molecules overlie the ice.

Because of the distance to Pluto, images obtained of the planet are small and blurred. Even the Hubble Space Telescope has difficulty seeing any detail on the planet's surface.

But between 1985 and 1990, astronomers were offered the opportunity to map the surface when Charon's orbit took it across the front of the planet every few days, as viewed from Earth.

Pluto may be similar to Triton

As Charon's shadow repeatedly obscured Pluto's disc and then moved away, astronomers were able to measure the variations in light from the planet to gather information about surface markings.

A team of scientists from the Southwest Research Institute in Boulder, Colorado, US, used the McDonald telescope in Texas to watch these so-called "mutual events" in several colours to construct the new map of Pluto's surface.

The dark region south of the equator may be similar to the dark streaks seen on Triton, Neptune's major moon. But astronomers will not know for certain until a space probe passes the tiny planet.

That will not happen until 2015 at the earliest. The map is published in the Astronomical Journal.